Volume of information keeps increasing along with the development of the information society. The development of a large capacity and a low-priced memory is demanded in accordance with the increase of the volume of information. Particularly, the magnetic disk to which the information access is done in the magnetic field is paid to attention as high density storage medium that is rewritable in information. The research and development for a magnetic disk unit which incorporates the magnetic disk and a head, and accesses the magnetic disk information by the head is done actively for making to a large capacity further.
The magnetic disk unit is often used in such a way that it is installed in electronic equipment such as personal computers and the server devices. The magnetic disk records thereon the system data composed of the serial number of the device, management information that indicates the defect area in the user area, the password for security, and the log of the access error, in addition to the user data that is the object of the information access. Usually, there is prepared the system area on which the system data is recorded in the most outer and inner of the magnetic disk, and the information access is executed for the system area when the password is changed by the user or the access error occurs, so that the system data is updated.
When the power supply of electronic equipment is turned on, the system data recorded on the magnetic disk is first read, and the password and management information, etc. are acquired, and here, they are used to activate the electronic equipment. Therefore, when the system data recorded in the system area is destroyed at the time when data is updated, the electronic equipment might not be able to be activated. Especially, in recent years, in order to improve the capacity of the magnetic disk unit, increasing TPI (number of tracks per an inch) of the magnetic disk is done. Narrowing the distance (track pitch) between the adjacent tracks causes the access error to the system area to occur easily.
As a method of solving such a problem, it is considered to apply the verify processing in which the data newly written is read after data is updated to confirm the presence of the error, and in the event that the error occurs, correct data is written again (For instance, refer to Japanese Laid-open Patent Publication No. 2004-319080). Moreover, it is widely performed that two or more system areas are prepared on the magnetic disk for the record of the system data, and the system data is recorded in those two or more system areas on a multiple basis. Applying multiplexing of these system data and verify processing makes it possible to improve the reliability of the data update, and in addition even if one system area is destroyed, it is possible that electronic equipment can be started by reading the system data recorded in another system area.
However, in a case where that both the multiplexing of the system data and the verify processing are simultaneously applied, it is necessary to execute both writing and reading of data whenever data is updated, and both the writing and reading of data are executed for each two or more system areas. Thus, the access time will increase.
In this respect, Japanese Laid-open Patent Publication No. 2006-309866 discloses technique in which identification data that identifies the medium is read, and ON/OFF of the verify processing is automatically switched in accordance with the identification data. Japanese Laid-open Patent Publication No. 2005-322287 discloses technique in which the access frequency to each two or more system areas is recorded, and when the system data is read, the system data recorded in the system area with the largest access frequency of two or more system areas is read. According to the technique disclosed in Japanese Laid-open Patent Publication No. 2006-309866, it is possible to omit the verify processing for the medium with high reliability of the data update. According to the technique disclosed in Japanese Laid-open Patent Publication No. 2005-322287, it is possible to save time that reads all of the two or more recorded system data on each of two or more system areas and compares those with one another, and is possible to acquire easily the latest system data of those two or more system data.
However, even if the above-mentioned technique is applied, it is impossible to cope with both the reduction of the processing time to the access to the system area and the improvement of reliability enough. Especially, there is a problem that electronic equipment cannot be started when failing in reading the system data to all system areas.
This problem doesn't limit to the magnetic disk unit that executes the information access for the magnetic disk, and be a problem of generally applying to the data memory that executes the information access in the recording medium where the system data was recorded in the multiple.